OK, I got some expert advice on the magnetic: The sytems use the WGM - the World Magnetic Model. There is an International Geometric Reference Field that is developed every 5 years and this model is fed into the data base of the inertial reference systems. The current model is for 2000 - 2005. More magnetic info:
www.ngdc.noaa.gov/seg/potfld/faqgeom.html